We propose to locate some 80 percent of the approximately 9,500 pairs of same-sex twins born in Minnesota from 1936 through 1955. These twins will be solicited to provide biographical (including medical) as well as personality and interest test data, using three successive levels of increased incentive. The first objective is to establish a large twin family registry that could be useful to ourselves and others in future psychiatric and biomedical research. The second objective is to test the hypothesis that easily recruited dizygotic twins are more similar within-pairs on many dimensions than are less easily recruited DZ twins and, hence, that published DZ correlations overestimate the true similarity of DZ twins generally. The same data will be obtained from spouses and 1st degree relatives of participating twins. The third objective is to replicate and extend previous findings of "emergenic" traits in which MZ correlations are high while DZ (and other 1st degree) correlations are low or zero. A fourth objective is to perform large-scale biometrical analyses on these twin-family data. A fifth objective is to provide a basis for selecting a truly representative sample of 100 MZ and 100 DZ pairs in which each twin has a spouse and at least one young-adult offspring available to come to our University laboratories for 2 days of aptitude and psychophysiological testing.